The present invention relates to a method of manufacturing a phase shift mask, and more particularly, to a method of manufacturing a halftone phase shift mask.
In a general mask using a chrome film, a light-shielding layer for shielding light is selectively formed on a quartz or glass substrate. In a part where the light-shielding layer is not formed, light is transmitted with a uniform phase, whereas in a part where the light-shielding layer is formed, light is not transmitted.
Accordingly, in such a mask, the destruction and interference phenomena occur in the edge parts of the light-shielding layer, which reduces the size of the actual light-shielding region. Thus, it is difficult to form a desired pattern using the general mask.
In order to solve the disadvantages of such a general mask, a phase shift mask has been developed recently.
The phase shift mask compounds the phase of light being transmitted through the mask to 180.degree. or 0.degree. along the pattern arrangement, thereby removing the destruction and interference phenomena in the edge parts of the general mask.
Furthermore, among such phase shift masks, a halftone phase shift mask useful for improving the resolution limit of a contact hole has been developed.
This halftone phase shift mask is a mask in which the thickness of a light-shielding layer is formed very thinly so that the light-transmissivity of the light-shielding layer is 4-30%. Further, the halftone phase shift mask includes a phase shift film adhered thereto for shifting the phase of the light transmitting therethrough to 180.degree..
Hereinafter, a conventional halftone phase shift mask will be described with reference to the attached drawings.
To begin with, as shown in FIG. 1A, a transmissive substrate 1 such as a quartz or a glass is prepared. On the transmissive substrate 1, a halftone pattern material layer 2 (Cr.sub.2 O.sub.5) of a predetermined thickness is formed to cause a phase shift.
At this time, the halftone pattern material layer 2 has a characteristic of shifting the phase of a light passing through to 180.degree. and transmitting only 5-10% of the incident light.
Accordingly, in the open region of the halftone pattern material layer 2, light transmits through the transmissive substrate 1 so that the positive intensity profile of light is achieved as shown in FIG. 1A.
On the other hand, where the halftone pattern material layer 2 is formed, only 5-10% of the incident light is transmitted with the phase of light shifted to 180.degree., thereby showing a negative value. That is, the intensity profile as shown in FIG. 1B appears. Expressing the intensity profile in its absolute value, a profile as shown in FIG. 1C is realized.
FIG. 2 shows a construction of a conventional contact hole mask of a general DRAM. It includes a cell part 11 in the central region, an overlay monitor key part 12 in the edge region, and an alignment key part 13.
Here, when the halftone phase shift mask is used according to the conventional method, the sidelobe of small contact holes located in the cell part 11 is worth a little consideration. However, since a larger exposure energy is required to form large contact holes such as the alignment key part 13 or overlay monitor key part 12, the sidelobe of such large contact holes, which is proportional to the exposure energy, becomes large.
That is, in order to form the overlay monitor key part 12 and alignment key part 13 having large contact holes, the open region of the halftone pattern material layer should be larger than the size of the large contact holes. Then, this requires a large exposure energy for the masking. However, an increase in the exposure energy causes an increase in the size of the sidelobe.
FIGS. 3A to 3D show the pattern formation of a photoresist for forming an overlay monitor key and an alignment key using the conventional masking method.
To begin with, as shown in FIG. 3A, a halftone pattern material layer 22 is formed on a substrate 21 and then, patterned in accordance with the overlay monitor key pattern or the alignment key pattern, respectively.
At this time, as shown in FIG. 3B, light transmitting through an open region having no halftone pattern material layer 22 has a positive phase, whereas light transmitting through the halftone pattern material layer 22 has a negative phase. Expressing the intensity profile of light by its absolute value is as shown in FIG. 3C.
Here, since the transmissivity of light in the halftone pattern material layer 22 is 5-10%, the intensity profile of light shows a higher intensity for the open region than for the halftone layer 22. However, since the pattern desired to form is the overlay monitor key or alignment key pattern, the sidelobe attended by the halftone pattern material layer 22 is increased.
This is because the exposure energy increases if the pattern desired to form is large. By using a large exposure energy to form a large pattern, the size of the sidelobe also increases. Consequently, as shown in FIGS. 3D and 3E, the pattern of the photoresist is formed inaccurately, and the sidelobe 23 appears around the overlay monitor key or alignment key pattern.
Accordingly, the aforementioned method of manufacturing using the conventional halftone phase shift mask has the following problems.
First, the sidelobe of the masking material becomes large so that the overlay operation cannot be effectively monitored. Second, consequently in the subsequent step, alignment becomes impossible.